French troops have launched their first ground assault against Islamist rebels in Mali in a broadening of their operation against battle-hardened al Qaeda-linked fighters who have resisted six days of air strikes.

France has called for international support against Islamist insurgents it says pose a threat to Africa and the West, acknowledging it faces a long fight against the well-equipped militant fighters who seized Mali's vast desert north last year.

After Islamist pledges to exact revenge for France's intervention, militants claimed responsibility for a raid on a gas field in Algeria.

Mauritanian media said an al Qaeda-linked group claimed to have seized as many as 41 hostages, including Americans and Europeans, in the attack, carried out in retaliation for Algeria allowing France to use its air space.

Severla people were killed.

French army chief Edouard Guillaud said his ground forces were stepping up their operation to engage directly "within hours" with the alliance of Islamist fighters, grouping al Qaeda's North African wing AQIM with Mali's home grown Ansar Dine and MUJWA militant movements.

Residents said a column of 30 French Sagaie armoured vehicles advanced toward rebel positions from the town of Niono, 300km from the capital Bamako. With the Malian army securing the northern border region near Mauritania, Islamist fighters were pinned down in the town of Diabaly.

"Fighting is taking place. So far it is just shooting from distance," said Oumar Ould Hamaha, a MUJWA spokesman. "They have not been able to enter Diabaly."

A Malian military source said French special forces units were taking part in the operation.

Guillaud said France's strikes, involving Rafale and Mirage jet fighters, were being hampered because militants were using the civilian population as a shield.

"We categorically refuse to make the civilian population take a risk. If in doubt, we will not shoot," he said. Residents who fled Diabaly said Islamists had used the town's inhabitants to protect themselves in recent days.

Many inhabitants of northern Mali have welcomed the French attacks against Islamists who have imposed a harsh form of sharia law, cutting off hands and feet for crimes, and destroyed the famed shrines of the ancient desert town of Timbuktu.

Residents said French fighter jets struck the headquarters of the Islamic police in Niafunke, a sleepy town on the Niger river near Timbuktu, as part of Operation Serval, named after an African wildcat.

Defence Minister Jean-Yves Le Drian acknowledged that France faced a difficult operation, particularly in Western Mali where AQIM's mostly foreign fighters have camps. Mauritania has pledged to close its porous frontier to the Islamists.

"It's tough. We were aware from the beginning it would be a very difficult operation," Le Drian said.

President Francois Hollande said French forces would remain in Mali until stability returned to the West African nation. Hollande said France hoped, however, to hand over to African forces in its former colony, "in the coming days or weeks."

West African military chiefs met for a second day in Bamako to hammer out details of a UN-mandated deployment which had been expected to start only in September but was suddenly kick-started by French intervention.

They said their aim was to send in the first detachments of a 2,000-strong emergency force on Thursday (Friday NZT).

"Nigeria is ready and Niger has its troops at the border and is just awaiting the green light," said Aboudou Toure Cheaka, special representative of the ECOWAS regional bloc.

Regional powerhouse Nigeria, which is battling the Boko Haram Islamist movement at home, has said the first 190 soldiers from its 900-strong detachment would arrive soon.

Military experts fear that any delay in following up on the French air bombardments of Islamist bases and fuel depots with a ground offensive could allow the insurgents to slip away into the desert and mountains, regroup and counter-attack.

The Africans will join some 1,700 French troops involved in the operation, part of a contingent expected to reach 2,500 soldiers. France is using Harfang surveillance drones to guide its strikes and also plans to deploy Tiger attack helicopters.

While many French troops come battle-hardened from Afghanistan, some regional African forces may need to adjust to desert combat far removed from the jungle terrain many are used to. A contingent of some 200 EU military trainers, led by a French general, is not expected before mid-February.

With African states facing huge logistical and transport challenges to deploy their troops, Germany promised two Transall military transport planes to help fly in the soldiers.

Britain has already supplied two giant C-17 military transport planes - larger than France's five C-135 planes - to ferry in French armoured vehicles and medical supplies.

The United States is considering logistical and surveillance support but has ruled out sending in US troops.

Hollande's intervention in Mali brings risks for eight French hostages held by AQIM in the Sahara as well as the 30,000 French citizens living across West Africa. A French helicopter pilot was killed on Friday, France's only combat death so far.

AQIM and Ansar Dine have vowed to take revenge for France's intervention on its interests around the globe.

The attack in Algeria, where AQIM has its roots, targeted the southern Ain Amenas oil facility, located close to the border with Libya and operated by a joint venture including BP, Norwegian oil firm Statoil and Algerian state company Sonatrach.

Security experts have warned that the multinational intervention in Mali could provoke a jihadist backlash against France and the West, and African allies.

US officials have warned of links between AQIM, Nigeria's Boko Haram and al Shabaab Islamic militants fighting in Somalia.

Al Shabaab, which foiled a French effort at the weekend to rescue a French secret agent it was holding hostage, said it had decided to execute him. France has said it believes the hostage was already killed during its botched raid.

The conflict in Mali raised concerns across mostly Muslim West Africa of a radicalisation of Islam in the region. In Senegal, a traditionally moderate Islamic country, President Macky Sall warned citizens to be vigilant for attacks.

"We must be on the watch in our towns and villages because infiltrations are taking place," he said in a speech on Tuesday. "You will hear foreign preachers talking in the name of Islam. You must denounce them to authorities."

The fighting in Mali, a landlocked state at the heart of West Africa, has displaced an estimated 30,000 people. Hundreds have fled across the border into neighbouring Mauritania and Niger in recent days.

"We were all afraid. Many young fighters have enrolled with them recently," said Mahamadou Abdoulaye, 35, a truck driver who fled from the northern Gao region of Mali into Niger. "They are newly arrived, they cannot manage their weapons properly. There's fear on everybody's face."